1. Field of the Invention
The present invention relates to a method of controlling a direct gasoline injection type internal combustion engine with a turbocharger, and more particularly to a control method for improving a combustion at a time of being operated under a high load, purifying an exhaust gas and improving a specific fuel consumption.
2. Description of the Related Art
In a lean burn engine as typified by a direct gasoline injection engine, for the purpose of improving a specific fuel consumption, in an operation area having a low engine torque, an internal combustion engine is operated under a state in which an air is excess in comparison with a theoretical air fuel ratio, that is, under a so-called lean burn state. In the case that the lean burn is executed, an air amount which is defined by a fuel amount for achieving a torque required by a driver and a predetermined air fuel ratio is limited by an air volume displacement of the engine. Accordingly, a range in which the engine can be operated on the basis of the lean burn which is advantageous in view of improving the specific fuel consumption is limited to the range in which the engine torque is low. It is possible to expand the lean burn area by making the air fuel ratio small, however, since a fuel injection amount is also increased, an air-fuel mixture around an ignition plug becomes too rich. Accordingly, there is a problem that an incomplete combustion is generated and a smoke is generated.
On the contrary, in JP-A-2000-248978, there is described a technique that an operable area in the lean burn is expanded by increasing the air amount. In the publication mentioned above, there is disclosed a technique that an amount of oxygen corresponding to an increase of the fuel injection amount is secured by executing a supercharging, and the air-fuel mixture having a suitable air fuel ratio can be formed around the ignition plug, thereby preventing the smoke from being generated due to the incomplete combustion.
In accordance with the technique described in the prior art mentioned above, the lean burn area can be expanded while inhibiting the smoke discharge, however, there is listed up a new problem that an NOx discharge amount is increased in proportion to the increased air amount. The lean burn engine such as the direct gasoline injection engine or the like is provided with an NOx catalyst which temporarily adsorbs the NOx in the exhaust gas so as to accumulate at a fixed amount, and thereafter purifies the NOx in accordance with an operation of a reducing agent such as CO, HC component or the like. In order to purify the NOx accumulated in the NOx catalyst, a rich spike control which temporarily makes the air fuel ratio rich and supplies the fuel (CO or HC component) not contributing to the combustion to the catalyst is executed at every fixed intervals. Since the rich spike control additionally injects the fuel which does not contribute to the combustion, the specific fuel consumption is deteriorated. Accordingly, it is possible to improve the specific fuel consumption by expanding the lean burn area in accordance with the supercharging, however, there is generated a problem that the specific fuel consumption is deteriorated by the increase of the number of rich spike control and the fuel injection amount which are caused by the increase of NOx discharge amount.
On the contrary, in recent years, there is proposed a compression ignition engine which ignites and burns a gasoline air-fuel mixture on the basis of a compression motion of a piston without using the ignition plug. The compression ignition engine has a potential capable of reducing an NOx discharge concentration to {fraction (1/10)} or less in comparison with the conventional gasoline engine which executes the ignition and the combustion by using the ignition plug. Accordingly, since the NOx catalyst is not required, it is possible to simplify an exhaust system, and it is also possible to remove the fuel deterioration element such as the rich spike control. However, since this engine does not have any ignition source, it is hard to control the ignition and the combustion in a wide operation area of the engine, and the technique has not been established yet. Further, since the operation area on the basis of the compression ignition is limited to a very narrow range having a low speed and a low load, a lot of efforts for expanding the area have been executed.
For example, in JP-A-2000-310120, there is disclosed a technique of changing the number of combustion during a cycle in correspondence to the load such as one combustion per four stroke or one combustion per two stroke, at a time of judging a self ignition combustion so as to expand the operation area. Further, in JP-A-2001-3800, there is disclosed a method of controlling an ignition timing by using a plurality of fuel injections as a trigger for ignition. However, none of the techniques are sufficient for the engine employed in a motor vehicle.
Accordingly, it is possible to expand the operable range on the basis of the lean burn which is advantageous in view of improving the specific fuel consumption in the direct gasoline injection engine, however, since the rich spike control is required in view of the structure of the exhaust system provided with the NOx catalyst, there is a problem that the specific fuel consumption is not improved in an actual traveling condition. Further, in the compression ignition engine which can construct the exhaust system requiring no NOx catalyst, there has not been established a technique which can achieve the low NOx combustion in a wide operation range. Accordingly, none of the techniques are sufficient in view of the engine system which can achieve both of the improvement of specific fuel consumption and the reduction of exhaust gas.